Some doctors are becoming increasingly concerned rival clubs are flouting rules on concussion and that the NRL doesn't appear to be doing anything to enforce its own rules.

The NRL last year implemented strict guidelines that stipulated any players concussed would not be allowed to play on and that clubs who didn't observe the rule would be sanctioned.

The NRL said it had issued breach notices, which range from formal cautions to fines and trainers being suspended, but wouldn't elaborate on individual cases.

Some club doctors have been critical of this, saying it's not transparent when salary cap issues or judiciary charges are made public, and doubt anyone has been fined.

NRL medical director Ron Muratore reportedly recommended two breach notices be handed down last season - one to the Wests Tigers after Benji Marshall was allowed to continue against the Dragons - but these were largely ignored and there have been two or three instances this season when players have continued despite being heavily concussed.

One involved Souths halfback Adam Reynolds, who was knocked out by Warriors winger Ngani Laumape but allowed to continue. Reynolds was given smelling salts by a trainer - he wasn't checked by a doctor - and was clearly groggy.

"They used smelling salts, which is an old boxing technique to get a guy concussed to wake up," a Sydney-based club doctor said. "There's no other reason to use it. It's almost a smoking gun, that one."

Warriors doctor John Mayhew was about 20m from the incident and amazed Reynolds was allowed to continue.

"I believe he was knocked out," Mayhew said. "If I was in a similar situation, I would have had great difficulty in allowing that player to play on.

Continued below.

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"We are disappointed by the lack of apparent action by the NRL on teams that flout the concussion rules. There are a number of clubs, and we are one of them, that stick to the rules but we are concerned that at other clubs player welfare is affected by not sticking to the NRL mandated laws."

The Warriors played the final eight minutes of their match against the Cowboys this season with only 12 players after Ben Henry was concussed and they had used up their 10 interchanges. They clung on to win 20-18 but were disadvantaged because they were following the rules.

Last season Tim Sheens suggested allowing teams to introduce an 18th man for a concussed player, especially if foul play was involved.

This received some support but, as one club doctor said, could be open to abuse in the same way some rugby teams flouted the blood bin rule.

"The NRL has considered the use of an 18th player to replace a player who has been injured by illegal play, however, it's something which will not be introduced in the short term," NRL football operations manager Nathan McGuirk said. "We have dealt with clubs in instances where we have had concerns and we will continue to do so when necessary."

Head injuries are among the most difficult to assess, but also some of the most serious. Concerns over concussion are by no means restricted to league with rugby, the AFL and American football among other sports grappling with the issue.

The Australian Rugby Union was recently in the firing line for allowing George Smith to play on despite being knocked out early in the third test against the Lions.

Rugby has a five-minute window to assess players suspected of having picked up a head injury. The AFL allows 20 minutes. League doesn't have an assessment window and doctors are not normally allowed on the field.

The NRL might be reluctant to issue breach notices because it is a grey area and it would effectively be questioning the integrity of a club doctor even though they weren't there to assess a player.

It became a topic of considerable debate last year in Australian Rules when the AFL fined North Melbourne $20,000 and rebuked its football department for hindering its probe into Lachie Hansen's concussion.

Neurologist Dr Rosamund Hill said it was paramount a concussed player was removed from the field or they faced "catastrophic consequences".

"It's black and white from the medical perspective," she said. " ... if you don't follow the guidelines carefully, you are placing the person's long-term brain function at risk."

Hard knocks

Benji Marshall (Rnd 3, 2012)

Marshall was left groggy in consecutive weeks, first by Dragons prop Trent Merrin and then Canberra's Jack Wighton, but played on.

Joe Picker (Rnd 8, 2013)

The second-rower left the field with suspected heavy concussion but came back later to help Canberra beat the Storm 24-20.

Adam Reynolds (Rnd 17, 2013)

Reynolds was flattened by Warriors winger Ngani Laumape and given smelling salts. He continued.